The reuse of needle syringes for intravenous injections has been recognized as a cause of the spread of communicable diseases. Particularly with the current spread of AIDS, it has become desireable to discourage the reuse of unclean syringes. Disposable plastic syringes are widely available because of their low cost and convenience in handling. Although they are intended to be discarded after a single use, they are in fact commonly reused without proper disinfection, particularly in the case of intraveneous drug users.
Prior techniques for preventing reuse of needle syringes include various arrangements for locking out the plunger of a syringe after it has been once loaded and depressed to the end of its travel to inject the contents of the syringe. For example, U.S. Pat. No. 4,731,068 to Hesse discloses a two-part lock construction having a band or sleeve assembled at the injection end of the syringe and dimensioned to be frictionally slidable along the inner wall of the syringe, and a spider element mounted in a fixed position on the plunger and having barb points engaged with the sleeve. When the plunger is first retracted, the spider element and sleeve travel toward the distal end of the syringe together with the plunger. When the plunger is depressed toward the injection end, the sleeve remains at the distal end, through frictional engagement with the inner wall, while the spider element travels toward the injection end with the plunger. If a second attempt is made to retract the plunger, the barbs of the spider element, now exposed to the inner wall in the absence of the sleeve, will engage the inner wall of the syringe and prevent a second retraction.
An alternative arrangement in the Hesse patent has the sleeve slidably supported on the plunger and engaged with the spider element having curved, spring-like prongs assembled at the distal end of the syringe. On the first retraction, the sleeve remains engaged with the spider element, and on the first depression, it is moved toward the injection end to expose the prongs of the spider element. An attempt to retract the plunger the second time will be prevented by engagement of the prongs with the plunger. Other devices, e.g. as shown in U.S. Pat. No. 4,781,684 to Trenner, U.S. Pat. No. 4,493,703 to Butterfield, U.S. Pat. No. 4,391,272 to Staempfli, and U.S. Pat. No. 4,367,738 to Legendre, provide for modifications to the plunger or syringe wall structure which allow only one-way movement of the plunger or which will lock or disable the plunger after a first depression.
Currently, a standard type of widely used disposable plastic syringe has a relatively simple construction of a cylindrical plastic wall formed with a closed injection end and an opposite open end, and a plastic plunger formed with a sealed plunger end, a knob end for depressing, and a stem in between the ends with an "X" shaped cross-section. The prior devices for preventing reuse of a syringe all involve structures which are built-in to the syringe at the time of manufacture. These devices cannot be retrofitted to the type of disposable plastic syringes which are already widely sold and commonly available. Moreover, their specialized construction would depart from that of the standard type of disposable plastic syringe, and would require retooling for manufacture and/or the marketing of a non-standard product.
Some types of lockout elements have been proposed which are insertable into a syringe without requiring modification of the plunger or syringe walls, and therefore can be retrofit into the widely-used type of standard plastic syringes. In the PCT publication of International PCT Application No. PCT/US87/02408 of Michael J. Free, which issued as U.S. Pat. No. 5,000,737 on Mar. 19, 1991, a flute-shaped element is insertable between a pair of adjacent longitudinal ribs of the plunger and the inner syringe wall, and has radially inwardly and outwardly facing sets of points pointing toward the open (proximal) end of the syringe. One set of barbed points engages the plunger ribs and the other engages the syringe walls for the lockout function. However, these types of insertable barbed elements have a tendency to slip from positive locking engagement when force is applied to move the plunger relative to the syringe wall, and/or to be defeated by tampering with the barbed points through the open (proximal) end of the syringe.